Field of the Invention
This invention relates to a method for producing gypsum board. More particularly, the invention relates to a method of employing gasification technologies and/or low BTU fuel sources in a gypsum board dryer.
Description of the Background Art
Nearly 100% of all gypsum board production facilities use natural gas to meet the thermal requirements of the board plant. In a few rare cases oil is used as the energy source. The thermal energy is used for drying gypsum feed material, calcining the gypsum to hemihydrate, and drying the manufactured gypsum board.
Several techniques are used for drying the feed material these include, heating while grinding, running it through a heated cage mill (synthetic gypsum), drying rock in a rotary mill and simultaneously drying, grinding and calcining in a large mill. All of these techniques use direct fire methods where hot combustion gases are delivered to the gypsum.
There are also several techniques for calcining the gypsum. One method uses indirect heating of the gypsum in a kettle. A second uses the same concept but additionally uses a combustion method where the hot combustion gases are delivered to the material. A third method was mentioned above where rock is dried, ground, and calcined in one step. Yet another method starts will fine particle synthetic gypsum and flash calcines it in a hot mill. Again in all cases natural gas is the fuel of choice and direct contact of the hot gases and the gypsum is one of the preferred methods.
In gypsum board drying direct fired natural gas burners are used to deliver heat directly to wet gypsum boards as they continuously pass through a dryer. Typically gypsum boards are 25-33% moisture at the “wet” end of the dryer and less than 1% moisture at the dry end. Almost all of the water contained in the wet board is evaporated out of the board through the heat delivered from the combustion of natural gas in a natural gas burner. There is direct contact of this gas with humidified recirculated gas which is drawn over the wet gypsum boards causing them to heat up and allowing the evaporation of the water. The evaporated water becomes humidity in the now re-circulating combustion gases. It has been found that certain RH's in the dryer sections improve evaporation rates and the product board quality.
In general gypsum board dryers have three to four zones, each with re-circulating gas streams and each with certain desired relative humidity and temperatures. The gas burners are used to control the temperatures. The relative humidity is control by either venting the gas or by sucking the gas through the entirety of the dryer to the vent. It should be noted that there are high levels of gas re-circulation in each zone of the dryers throughout the entire dryer depending upon the design. Dryers which achieve high temperatures associated with high humidity's in their vents are typically the most efficient.
Thus, as noted, the production of gypsum board is energy intensive. Most of the energy is supplied from natural gas. The natural gas is burned to produce thermal energy required for drying the gypsum board. Modern gypsum board plants require around 200,000,000 BTU/hr. or approximately 200,000 cubic ft. of natural gas per hour. This is a tremendous amount of natural gas.
The large amount of natural gas required is problematic due to the tremendous instability in the natural gas market. High demand in 2005 to 2006 caused gas pricing to nearly quadruple in a few years. Last year gas pricing peaked at about $13.50/million BTU. By contrast, energy prices for coal/pet coke and biomass were generally less the $2.40/million BTU. Biomass has an additional advantage in that many conservation minded communities are now charging tipping fees to encourage the beneficial use of yard wastes and tree trimmings.
In view of the foregoing, there exists a need in the art for methods that use alternative energy sources or that make more efficient use of existing energy sources. One example is illustrated in U.S. Pat. No. 2,677,237 to Voysey. Voysey discloses a gas turbine power plant utilizing a sold water bearing fuel. The plant includes a number of fuel driers. The driers are in communication with a combustion chamber that, in turn, is in communication with a heat exchanger. The driers function in evaporating the water content from the fuel. The evaporated water can be heated via the heat exchanger prior to delivery into the combustion chamber.
Likewise, U.S. Pat. No. 5,253,432 to Raiko et al. discloses a drying method and dryer for use in a power-plant. More specifically, Raiko concerns a method for drying a water-consuming material in a power-plant process. Steam is collected from a dryer zone and passed to a combustion chamber or gasification device. The combusted steam is then utilized in a compressor and gas turbine.
Although the above reference inventions each achieve their respective objective, there continues to be a need in the art for a method of drying gypsum board that eliminates or minimizes the use of natural gas.